I use 100% pure organic oregano oil in my chickens fermented feed when they are under stress or if they are showing signs of any illness or any skin injuries. I add one to two drops per bird per feed. I get the oil from https://www.zanehellas.com/
I also use it for myself and my family in place...
I gave my sick australorp 1 drop of oregano oil mixed with her fermented feed with a little black pepper (the black pepper helps the body to assimilate the oil better) There are 20 drops in 1 millimeter.
Mix it in the feed throughly so it gets properly dispersed.
I find this method easier than...
oregano oil is best given 1 drop per food serving in wet mash or in fermented feed as prevention, 2 drops if the bird is sick. Pure oregano oil does not mix with water (no pure oil will) it will just stay as a blob floating on the surface and the chickens won't get any benefit from it at all.
with regular pharmaceutical grade droppers 20drops= 1 ml, so to get the dose people here on this thread have been recommending you'd need 0,6ml per gallon so 12drops.